The two biggest U.S. food-distribution companies announced a merger that will create a giant with about $65 billion in annual revenue and enhanced clout over purchasing by institutions ranging from restaurants and hotels to hospitals and schools.

Sysco Corp. said it will buy rival US Foods for $3.5 billion, uniting two middlemen that are already central players in the service economy. Sysco alone has about 425,000 customers, and it and US Foods together collected about 27% of the revenue in the U.S. food-distribution...